(Industrial Field of the Invention)
The present invention relates to a method of inspecting bonded wafers and especially relates to a method which is adapted to detect fine unbonded regions at the interface between the bonded wafers without fracture or destruction.
(Statement of the Related Art)
Thermal diffusion, epitaxial growth, and the like have hitherto been widely employed as technical methods of introducing impurities into a semiconductor material body in the manufacture of an electronic semiconductor device. Such prior art methods are employed because they have been well established from a technical point of view. However, when an attempt is made to produce a high power device by using one of the prior art methods, a technical limitation is imposed on the collector diffusion effected by thermal diffusion, or on the formation of a high-resistance layer of 100 5/8cm or more effected by epitaxial growth. This makes it difficult to provide a high power device of high voltage with large capacity.
Further, in the field of semiconductor integrated circuits in particular, a dielectric isolation technique adapted to isolate individual elements by means of dielectrics is excellent in respect the the parasitic capacity and isolation characteristics, but undesirably makes the substrate excessively large in warp. For this reason, the dielectric isolation technique involves a great disadvantage from a manufacturing standpoint.
Further, almost no attention has hitherto been paid to a method of bonding together the polished mirror surfaces, either directly or with an oxide film interposed therebetween, of two silicon wafers. However, much attention has lately been paid to a method of manufacturing a substrate for the above-mentioned high power device, or a substrate for the above-mentioned dielectric isolation. That is to say, in any application, the silicon wafer mirror surface bonding method can remarkably solve the above-mentioned drawbacks of the prior art.
The existing silicon wafer bonding method includes a method of merely superposing one silicon wafer mirror surface upon the other at room temperature in an ordinary air atmosphere, and heating the superposed wafer body at high temperature, for example, of 1,100.degree. C. for about two hours in a specially mixed atmosphere of oxygen and nitrogen gases bearing the ratio of 1:5 and a method of, in the case where an oxide film exists in between the method involves, applying a direct current or alternating current voltage across the wafers at the time of superposing them one upon the other so as to utilize electrostatic force acting between the wafers and then heating the superposed wafers in an atmosphere of nitrogen gas.
The technical problem concerning the mutual bonding of two silicon wafers resides in the respect that some unbonded regions are left between the opposing surfaces of the two mirror surface wafers, as a result of insufficient proximity, and that in consequence what is usually called "voids" are formed. In order to prevent the formation of such voids, studies have been made on the causes of formation of them. The causes can be considered to be dust particles or contaminations, or scratches having attached onto, or having been caused to the surfaces of the wafers. In particular, attention is drawn toward dust particles as the greatest cause of creating such voids.
The present inventor, however, has experimentally confirmed that it is impossible to completely prevent the formation of voids even by removal of all of the above-mentioned causes.
Further, as an improved silicon wafer bonding technique, Japanese Patent Unexamined Publication No. 61-182216, for instance, proposes a method wherein a gas which is easy to pass through or to be absorbed by the semiconductors is used as an atmosphere at the time of bonding wafers together to prevent such voids. Once a gas has been held in the interface between the bonded surfaces of silicon wafers, however, removal of such gas by penetration and absorption thereof is very difficult in actuality.
As another void preventing method, Japanese Patent Unexamined Publication No. 63-93107, for example, proposes a method of activating the bonded surfaces by irradiation of microwaves. This method, however, has a drawback in that it requires the use of a special system for irradiation of microwaves.
If the bonded surfaces of the wafers form a boundary or interface of an active region of a device, it is a problem whether a complete epitaxy is formed or not at the bonded interface. In a general bonded interface utilizing method, however, the bonded surfaces are indeed made to come close to the active region but consideration is given so that they may not come into the same. Accordingly, making the bonded interface completely monolithic is ordinarily not required.
However, there is a likelihood that, in case the bonded surfaces come close to the active region, the defective bonded interface sometimes causes the active region to have a crystalline disorder or a physical imperfection. Further, there is also a likelihood that the defective bonded interface causes performance characteristics of a power transistor, for instance, to be deteriorated. To eliminate such a likelihood, it is desireable to substantially eliminate the unbonded regions in the bonded interface and to provide a void-inspection method which is effective in substantially eliminating them.
In general, inspection of the bonding condition is conducted by utilizing an infrared transmission image. In this method, infrared rays are irradiated onto the bonded wafers from a direction perpendicular to the interface of the bonded wafers, to thereby facilitate observation a transmission image as obtained. Unbonded regions are observed as white image portions while bonded regions are observed as black image regions. When infrared radiation has been incident upon the bonded surfaces of wafers substantially at right angles thereto, if there exists any unbonded region in the bonded interface, reflection, absorption and a change in refractive index take place at such an unbonded region. For this reason, the transmitted amount of light is reduced as compared with that which is obtained from the bonded regions. The lightness and darkness of a transmission image are reversed one to the other depending upon the method of observation.
However, it has been experimentally proved that this method fails to detect any unbonded regions unless the gap between the bonded surfaces at the unbonded region is considerably large in dimension, for example, 0.1 .mu.m or more.
The unbonded regions each of which has a gap or clearance of a below 0.1 .mu.m can be observed only by suitably cutting the bonded wafers and observing the cut surface thereof with use of SEM (Scanning Electron Microscope). This method of observation, however, is a destructive inspection and, in addition, fails to observe the unbonded regions which are not included in such a cut surface.
For the above-mentioned reasons, there has eagerly been desired the development of an easy and nondestructive inspecting method of bonded wafers to determine whether the bonded interface is perfectly bonded or not.